Do you have "Optimize Mac Storage" checked in Photos>Preferences>iCloud? If so, then your library doesn't have full images of all your pictures, so you can't just drag and drop it. Probably the best plan is to close then option-click Photos and Create a new library located in the Pictures folder on you mac. Then, in Photos>Preferences>General make this new library your System Library. Then in Photos>Preferences>iCloud check iCloud and "Optimize Mac Storage." Then wait a few days until all your pictures are synced with iCloud.
If you don't have "Optimize Mac Storage" checked, then all your images are contained inside the library (and it takes up a lot more space on the drive), so you can just drag it over to the new location on you internal drive, double click on it, and start work. Remember to go to Photos>Preferences>General and make this new library your System Library. Then in Photos>Preferences>iCloud check iCloud, and check "Optimize Mac Storage" if you want to save space.
This is the opposite of what you asked, but the guidelines might help:
Move your Photos library to save space on your Mac - Apple Support